1. Field of the Invention
This invention relates to a pathlength controller for a ring laser gyroscope and, more particularly, to an improved pathlength control assembly having certain thermally expansive components to allow the controller to achieve a relatively wide range of axial displacement to provide the greater range of dynamic mirror movement needed for a multioscillator ring laser gyroscope.
2. Description of Related Art
Ring laser gyroscopes are an alternative form of rotation sensors which do not require the use of a spinning mass characteristic of a mechanical gyroscope. A ring laser gyroscope employs a Sagnac effect to detect rotation optically, as an alternative to the inertial principles upon which a mechanical gyroscope operates. Planar ring laser gyroscopes, of both triangular and square geometries, have been used in inertial navigation systems and flight control systems regularly in both commercial and military aircraft. The primary advantage of the ring laser gyroscope over the spinning wheel mechanical gyroscope is its ability to withstand relatively large mechanical shock without permanent degradation of its performance. Because of this and other features the expected mean time between failures of most RLG inertial navigation systems are several times longer than the mechanical gyroscopes they replace. The planar ring laser gyroscope was a first attempt at a non-mechanical truly strap-down inertial navigation system.
The earliest developed ring laser gyroscopes have two independent counter-rotating light beams or other electromagnetic propagation which travel within an optical ring cavity. In an ideal model of the ring laser gyroscope, these two light beams propagate in a closed loop with transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. However, when one steps away from the ideal model of two mode ring laser gyroscope operation, various sources of inaccuracy are observed. Among these inaccuracies in rotational sensing of a two-mode planar ring laser gyroscope is the phenomenon known as frequency lock or mode locking. Reflections and backscatter from the intra-cavity element and instabilities of the magnetic field associated therewith cause difficulties such as mode locking that need to be overcome in order to build a fully optical navigational grade multioscillator ring laser gyroscope. Mode locking is a major difficulty at low rotation rates where the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that of where lock-in occurs, and is then decreased, the frequency difference between the beams disappears at a certain input rotation. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is generally called the dead band of the ring laser gyroscope. Lock-in arises from the coupling of light between the beams. One means of overcoming the lock-in effect of the counter-propagating modes of light within a two mode gyroscope is to mechanically dither the mirrors or body of the gyroscope. This technique is known as rate biasing or mechanical dithering and prevents counter propagating waves from locking at low rotation rates. A more detailed explanation of the problems associated with a planar two mode gyroscope are described in Laser Applications, edited by Monte Ross, pages 133-200 (Academic Press, 1971).
Even the most effective mechanically dithered ring laser gyroscope adds a noise component to the output of the ring laser which in turn reduces its ultimate accuracy. Also, the presence of mechanical dither, either is mirror or full bodied dither, detracts from the desired goal of a fully strapped down inertial navigational unit. Since one of the primary benefits of a ring laser gyroscope is that it overcame the need for mechanical or moving parts, a body dithered planar two mode gyroscope does not truly meet this goal. In an effort to achieve a fully optical ring laser gyroscope, the non-planar multi-mode ring laser gyroscope was developed to overcome the effects of mode locking without the need to dither. The term (multioscillator) refers to four modes of electromagnetic energy that propagates simultaneously in the cavity as opposed to the usual pair counter-propagating linearly polarized modes that exist in the conventional two mode gyroscope. A detailed discussion of the operation of the multi-oscillator laser gyroscope is presented in the article entitled "Multioscillator Laser Gyros" by Weng W. Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics, Vol. QE-16, No. 9, September 1980. An example of this theory of multioscillator ring laser gyroscope may also be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor); and U.S. Pat. No. 4,813,774 entitled "SKEWED RHOMBUS RING LASER GYRO" issued Mar. 21, 1989 to Raytheon Corporation (Terry A. Dorschner, et. al., inventor).
With reference to FIGS. 1A and 1B, the basic multi-oscillator ring laser gyroscope has an optical path 10 formed between four mirrors 12, 14, 16 and 18. Mirrors 16 and 18 are generally fixed, and one of these mirrors may be semi-transparent in order to allow light to leave the resonator and fall upon photodetectors (not shown and external to the path) for signal processing in order measure rotation of the gyroscope. When the signals are subtracted during the electronic processing to remove the Faraday bias, the scale factor of the gyroscope is doubled over the conventional ring laser gyroscope. At least one of the other mirrors 12 and 14 are transducer driven mirror assemblies which are used to effectuate pathlength control. A Faraday element 15 is also present in the optical path 10 in order to effectuate non reciprocal splitting of pairs of left circularly polarized (LCP) and right circularly polarized (RCP) light beams. The multioscillator ring laser gyroscope contains the two gyroscopes (GYRO 1 and GYRO 2 of FIG. 1B) symbolized by their respective gain lines 22, 24, 26, and 28 under the atomic spectra resonant gain profile 20. Reciprocal splitting between left circularly polarized (LCP) and right circularly polarized (RCP) light beams is accomplished by the non-planar geometric configuration of the mirrors 12, 14, 16, and 18, shown in an exaggerated form in FIG. 1A as a quadrilateral optical path split (the broken line connecting mirrors 14 and 16). The multioscillator ring laser gyroscope uses the Faraday element 15 within the cavity (or, alternatively, a magnetic field on the gain plasma) to provide a phase shift between the counter propagating waves to prevent mode locking. With reference to FIG. 1B, the non-planar ray path reciprocally rotates the polarizations of the counterpropagating light beams by many degrees yielding the necessary high purity circular polarization. This splitting is known as reciprocal splitting and typically is in the range of 100 MHz. By placing a Faraday element 15 in the beam path of a nonplanar ring laser gyroscope, and when the proper magnetic field is applied to the Faraday glass element, nonreciprocal splitting of each gyroscope is achieved. At least four modes are produced: a left circularly polarized anti-clockwise frequency 22 (L.sub.a), a left circularly polarized clockwise beam 24 (L.sub.c), a right circularly polarized clockwise beam 26 (R.sub.c), and a right circularly polarized anticlockwise beam 28 (R.sub.a). The Faraday splitting between clockwise and anti-clockwise modes is about 1 MHz.
Although a multioscillator ring laser gyroscope provides a strap-down method of providing rotation measurement which is not subject to low rotation rate mode locking and therefore needs no dither mechanism, all ring laser gyroscopes are prone to optical pathlength changes due to thermal expansion of the gyroscope frame. Therefore, the optical pathlength of the gyroscope must be controlled and monitored to make certain that the resonant cavity operates at the same gain line of the atomic spectra gain curve. Due to the multiplicity of their applications, ring laser gyroscopes are required to operate over a wide temperature range, such as -55.degree. C. to +70.degree. C. Since the laser light beam emitted by the active gain region of the gyroscope propagates around the ring laser by means of reflection off the surfaces of at least 3 mirrors, thermal expansion of the frame and mirrors will cause a significant change in cavity resonant wavelength. It is therefore necessary to provide a pathlength control mechanism to slightly vary the optical pathlength of the gyroscope ring resonator in order to preserve the fundamental resonance of the cavity to which all sensing instrument components of the gyroscope are calibrated. Even where low expansion glass materials are used for building a monolithic frame which supports to optical cavity path between the mirrors, the pathlength of a ring laser gyroscope will still experience a substantial change in path length during temperature changes. This change can be as much as 5 wavelength or more at the resonant frequency of the light produced by the gaseous active medium, such as a helium-neon mix. In an active path length control system, the changes in pathlength due to thermal expansions and contractions are monitored by detector electronics and provide feedback information for driving one or more piezo-electric transducers. However, the standard active pathlength control assembly does not provide sufficient axial movement of the mirror surface over a sufficient range to accommodate the dynamic changes due to temperature found in a multioscillator ring laser gyroscope.
The applicants are aware of certain disclosures by Raytheon Corporation of Lexington, Mass. directed to a Passive Pathlength Control for Ring Laser Gyroscopes and a High Performance Hybrid Pathlength Control, as well as Laser Pathlength Tuning Elements, which are the subject of a United States Patent Application entitled Passive Pathlength Control Mirrer For Laser filed Dec. 18, 1990, as Ser. No. 07/630,213. The assignee of this application is a Licensee of these disclosures from Raytheon Corporation. These disclosures (which were written before the conception of the present invention by the applicants herein) are directed to a passive pathlength control assembly which performs pathlength control solely by making use of thermally expansive materials to achieve such control. The Hybrid Pathlength Control disclosure suggests the use of at least one such passive controller and at least one other piezo-electric active controller for the same optical path of a multioscillator ring laser gyroscope instrument.